This invention relates to a curable coating composition particularly adapted for use as a clear top coat for automobiles which has fast drying speed. More particularly, the type of coating involved is that which includes as the primary binder constituents an acrylic polyol and a curing agent, typically a polyisocyanate.
Automobiles are sometimes coated with a plurality of layers, typically a pigmented base coating composition onto which is subsequently applied a different top coating composition. The top coat is frequently referred to as "clear," that is, substantially without pigment, but minor amounts of pigments or other materials that affect the appearance of the automobile may be included. Acrylic polyol based top coats are particularly useful for their compatibility with a wide variety of base coating compositions.
An important factor in the productivity of an automobile painting operation is the speed with which the top coat dries to a condition that is no longer tacky. "Drying" in this context does not entail completing the curing reactions, but rather an initial surface hardening due primarily to solvent evaporation. Although acrylic polyol based coating compositions have a desirable combination of properties, it has been desired to shorten the dry time of this type of coating. It was known to increase drying speed by selecting monomers that increase the T.sub.g (glass transition temperature) of the acrylic polyol. In particular, increasing the methacrylate monomer content of the acrylic polyol was known as a way to increase T.sub.g (U.S. Pat. Nos. 5,279,862 and 5,314,953). Some attempts to further increase drying speed, especially under ambient conditions, have heretofore not been as successful as desired because increased drying speed has generally been accompanied by an undesirable degradation in the appearance of the final coating. A more successful approach to increasing cure speed while maintaining other important properties is the subject matter of commonly owned, co-pending U.S. patent application Ser. No. 09/086,919 filed May 29, 1998. It would be desirable to further improve upon the compositions disclosed in that application by reducing the volatile organic content.
Reducing the amounts of volatile organic solvents in automobile refinish coatings is challenging because the high appearance standards for automotive coatings are particularly difficult to achieve with coating compositions having reduced amounts of organic solvents. For use as an automotive refinish coating, there is the further requirement that the coating composition be capable of ambient temperature cure or, at most, a forced heating step with a slightly elevated temperature. Automotive refinish clear coatings have the additional requirement that the clear coating should not exhibit a significant amount of yellowing since this can detract from the appearance of the underlying pigmented coating.
U.S. Pat. Nos. 5,279,862 and 5,314,953 (Corcoran et al.) disclose fast drying auto refinish coating compositions that rely upon a high methacrylate monomer content in the acrylic polyol portion of the binder resin. Reducing organic volatiles is not an objective of these patents.
U.S. Pat. No. 5,514,755 (Fenn et al.) and U.S. Pat. No. 5,741,880 (Valpey et al.) are directed to reducing organic volatile content of auto refinish coatings. To attain higher solids content compositions, lower molecular weight polymers are the objective of these patent. In particular, the patents discloses the use of acrylic polyols made from monomers that include substantial portions of isobornyl methacrylate.
U.S. Pat. No. 5,753,756 (Aerts et al.) also deals with reducing the volatile content of auto refinish coatings, wherein the approach is to use a complex hydroxy functional oligoester in the binder resin. The use of caprolactones is mentioned as part of the synthesis of these oligoesters (column 4).
French Patent Publication No. 2 452 500 discloses generally the use of glycidyl compounds reacted with acrylic acid or methacrylic acid as a component of acrylic polyols.
Accordingly, there is a need for a fast drying automotive refinish clear coating composition which not only has excellent physical properties and appearance, but also meets increasingly stringent requirements for lower volatile organic content.